Objective: To determine the efficacy of L-carnitine therapy in selected cases of male factor infertility. Design: Placebo-controlled, double-blind, crossover trial. Setting: University tertiary referral center. Patients: One hundred infertile patients (ages 20-40 years) with the following baseline sperm selection criteria: concentration, 10-20 x 10(6)/mL; total motility, 10%-30%; forward motility, <15%; atypical forms, <70%; velocity, 10-30 micro/s; linearity, <4. Eighty-six patients completed the study. Interventions: Patients underwent L-carnitine therapy 2 g/day or placebo; the study design was 2 months of washout, 2 months of therapy/placebo, 2 months of washout, and 2 months placebo/therapy. Main Outcome Measures: Variation in sperm parameters used in the patients selection criteria, in particular, sperm motility. Excluding outliers, a statistically significant improvement in semen quality, greater than after the placebo cycle, was seen after the L-carnitine therapy for sperm concentration and total and forward sperm motility. The increase in forward sperm motility was more significant in those patients with lower initial values, i.e., <5 x 10(6) or